During our investigations on an improvement of useful antibiotics, istamycins A and B, which we already produced by fermentation of a new microorganism, Streptomyces tenjimariensis FERM P-4932 (ATCC No. 31603) (Japanese Patent KOKAI No. 145697/80, "Journal of Antibiotics" 32, 964-966 (September 1979); U.K. patent application No. GB 2048855A published Dec. 17, 1980), we noticed an improved antibacterial activity of formimidoylfortimicin A [SF-2052 substance; Journal of Antibiotics, 32, 1354-1356 (1979)] over fortimicin A and tried to apply our finding to istamycins, that is we intended to convert the amino group in the glycine moiety of istamycins A and B to an amidine group, and thus we have now successfully produced new compounds, formimidoylistamycin A and formimidoylistamycin B, which have an antibacterial activity higher than that of fortimicin A and of the same level as those of istamycins A and B and which are of lower toxicity than those of istamycins A and B.